Multichannel sound reproduction systems such as Dolby Pro Logic or Dolby Digital (Dolby, Dolby Pro Logic and Dolby Digital are trademarks of Dolby Laboratories Licensing Corporation) require, for example, five speakers, placed at particular locations and particular angles. This can be costly and space consuming. It would be desirable to have surround sound without rear loudspeakers, to save on cost and space. However, conventionally, front loudspeakers only provide front sound images.
It is known to process multiple channels representing sounds from many directions, and combine them into two signals for reproduction over headphones, retaining the apparent multiple directions. With headphone reproduction the left signal goes to the left ear, and the right to the right, with no crosstalk. Sounds can appear to come from the sides of the listener as well as from the front, or in some cases the rear.
Considering each of the multichannel inputs as representing sound from a particular direction, such processing for headphones typically includes at least applying appropriate HRTFs (head related transfer functions) to each input to simulate the paths from its desired apparent direction to the two ears, so that the headphone listener perceives each channel as coming from the desired direction. Such headphone processors, which provide two outputs in response to more than two inputs, are referred to by a variety of names such as “multi-axis binaural steering” processors, “multi-channel binaural synthesizers”, “headphone virtual surround” processors, and the like. Some headphone processors also provide processing in addition to applying directional HRTFs, such as adding simulated reflections and/or artificial ambience to one or more of the channels. All such processors, whether employing only directional HRTFs or also additional processing, such as artificial reflections and/or ambience, are referred to herein as “headphone processors.” Some examples of headphone processors include those described in published International Application WO 99/14983 (designating the United States) and in U.S. Pat. Nos. 5,371,799; 5,809,149; and 6,195,434 B1. Each of said application and patents are hereby incorporated by reference, each in their entirety.
Conventional two-channel stereophonic material is intended for reproduction over two loudspeakers. Each of the listener's ears receives sound from both loudspeakers, with, of course, different path lengths and frequency responses. In other words, there is acoustic crosstalk. In general, all sounds so reproduced appear to lie within the space between the loudspeakers.
It is also known to modify signals prior to application to two loudspeakers to cancel the acoustic crosstalk, at least partially. This allows the apparent position of sounds to lie well outside the space between the loudspeakers, and is the basis of “virtual surround” processes. To the extent that the crosstalk is cancelled, the sounds entering the ears from the two loudspeakers resemble those provided by headphones, i.e., without crosstalk. Crosstalk cancellers (sometimes referred to as “spatializers” or “panoramic processors”) are well known in the art, dating at least from U.S. Pat. No. 3,236,949 (Atal and Schroeder), which patent is hereby incorporated by reference in its entirety. A computer-software-implemented acoustic-crossfeed canceller using very low processing resources of a personal computer is disclosed in U.S. patent application Ser. No. 08/819,582 of Davis et al, filed Mar. 14, 1997, which application is hereby incorporated by reference in its entirety.
As is also known, signals representing multiple channels, including sounds originally coming from outside the space between the loudspeakers can be processed as if for reproduction over headphones and then fed via an acoustic crosstalk canceller to two front loudspeakers arranged in a conventional stereo configuration, such as at the sides of a computer monitor or a television picture tube. This combination of headphone processing and crosstalk cancellation allows the apparent position of sound sources to lie to the sides, or in some cases the rear, using only a pair of front loudspeakers.
FIG. 1 is a schematic block diagram showing a prior art arrangement in which the multiple channels of a multichannel source, such as a five-channel source (each channel representing a direction, such as left front, center front, right front, left surround and right surround), are applied to a headphone processor 2. The two outputs of the headphone processor are applied to a crosstalk canceller 4, which also has two outputs. One output of the crosstalk canceller is applied to a first loudspeaker 6 and the other output is applied to a second loudspeaker 8.
The combination of headphone processing and crosstalk cancellation feeding a pair of loudspeakers is superior to a crosstalk canceller alone because the processing for headphone reproduction introduces additional directional cues by introducing directional HRTFs (crosstalk cancellers may include only “one ear to the other” HRTFs) and, in some headphone processors, simulated multiple acoustic paths (including reflections) between apparent image positions (outside the loudspeakers) and the listener's ears. Thus, with combined headphone processing and crosstalk cancellation, virtual sound images may appear not only at the sides of a listener's head but also from further back.
However, there are disadvantages of such a combined headphone processing and crosstalk cancellation scheme. The front sound channels deft front, center front, right front) of the multichannel source are intended to be reproduced over loudspeakers and are satisfactorily reproduced by two loudspeakers that reproduce the left front and right front channels and also provide a virtual or “phantom” center front image (provided, of course, that the listener is appropriately located with respect to the two loudspeakers). Consequently, processing the front sound channels is not necessary and should be avoided (in accordance with the “least treatment” principle). Headphone processing of the front channels involves at least the application of directional HRTFs that may cause colorations or changes in timbre, for example. Other headphone processing techniques, for example the simulation of reflections or reverberation, may introduce other noticeable and unnecessary alterations of the front channel signals or may produce artifacts. Crosstalk cancellation may also adversely affect the front channels. Crosstalk cancellation is most effective when the playback environment, the listening room, introduces little by way of reflections. Consequently, in practical “real listening room” applications, crosstalk cancellation is incomplete. Thus, even if headphone processing of the front channels were transparent, the subsequent crosstalk cancellation in prior art of the type shown in FIG. 1 would likely impair the reproduced front channel sound.
In accordance with the present invention, impairment of the front channel reproduction is avoided while retaining the benefits of improved surround channel reproduction from a pair of loudspeakers.